Gale Warning 447, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
451 AM AKDT Wed Aug 23 2023

.ANALYSIS AND UPPER LEVELS...

High pressure sits over Southcentral and much of the Gulf of
Alaska this morning. One more day of generally clear skies is on
tap for Southcentral before the pattern becomes more Fall-like
later this evening into tomorrow. To the west a longwave trough is
becoming established over the Aleutians with several shortwaves
in the process of moving through its ageostrophic flow. Water
vapor imagery depicts the more immediate of the shortwaves lifting
across Bristol Bay this morning. Increasing light showers are
noted across the Kuskokwim Mountains, which are expected to drift
over the Kuskokwim Delta through tonight. A second shortwave
exists to the south of the Central Aleutians and will amplify into
a deeper Bering Sea low on Thursday. Finally, a third shortwave
is tracking south of Kamchatka. This latter shortwave is expected
to amplify south of the Aleutians, eventually bringing even more
rain into the Prince William Sound region over the weekend.

&&

.MODEL DISCUSSION...

Models remain in surprisingly good agreement both in the short
term and the long term. Rain chances are forecast to increase
through the weekend as multiple systems move across Southwest and
Southcentral Alaska. The biggest questions entail rainfall amounts
for Southcentral in the long term, specifically over the weekend.
Deterministic and ensemble models are advertising the development
of an atmospheric river into Prince William Sound Saturday and
Sunday. There is the potential for 72 hour rainfall amounts to
exceed 5 inches across eastern Prince William Sound, though we are
still several days away.

&&

.AVIATION...

PANC...VFR conditions and light winds will persist through this
afternoon. A Turnagain Arm jet will then develop this evening in
response to the storm system moving into the eastern Bering Sea.
This jet will continue to strengthen through Thursday. The
terminal will be positioned on the northern edge of this jet, with
the core of strongest winds crossing Fire Island. Once gusty
southeast winds move into the terminal tonight, they will persist
through at least Thursday night, with the strongest winds expected
Thursday afternoon and evening. Meanwhile, rain will develop after
12Z Thursday. Deep and moist southerly flow above the
southeasterly Turnagain winds should easily drop ceiling and vis
down to MVFR sometime Thursday morning.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 through 3/Today
through Friday night)...

Today will be the last of a strong of warm and dry conditions across
Southcentral Alaska before a change to a rainy pattern - very
typical for late August. Aside from localized areas of low clouds
and fog this morning, the day will start out with a fair amount of
sunshine across the region with an upper ridge overhead. A short-
wave trough over Bristol Bay and the eastern bering Sea this
morning will ride overtop of the ridge this afternoon bringing
increasing clouds and showers primarily to the Susitna Valley and
Talkeetna Mountains.

Meanwhile, a gale force low of tropical origins will track into
the eastern Bering Sea this afternoon then continue northward off
the coast of western Alaska tonight through Thursday. A new ridge
will rapidly build out ahead of it, setting up shop from the
northeast Pacific to western Canada - and leading to a quasi-
stationary atmospheric river pointed at southern Alaska through
the weekend. With this first low so far west of Southcentral, the
dynamical forcing acting on the tropical airmass will be fairly
weak. Still, deep south to southwest flow with embedded short-
waves aloft along with a weakening surface occluded front will
bring widespread rain to Kodiak Island and Southcentral tonight
through Friday. The heaviest rain will be in upslope flow in the
Prince William Sound region as well as the northern Susitna
Valley. The track of the low out west will favor gusty southeasterly
gap winds, including Turnagain Arm into portions of Anchorage,
the Knik River Valley into Palmer, and along the Copper River
Valley into Glennallen.

River levels are starting out fairly low, so do not expect any
flooding issues with this first round of rain. However, the flood
threat will increase this weekend into early next week as the
atmospheric river remains pointed at Southcentral Alaska and
multiple storms/troughs transit the region. Stay tuned to the
latest forecasts and updates on the potential for flooding this
weekend.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA/BERING SEA/ALEUTIANS (Days
1 through 3)...

A North Pacific low moves into the southern Bering Sea today,
resulting in the development of gusty winds and widespread steady
rainfall along the front. Winds and rain will continue to spread
northward across the central/eastern Bering and Southwest Alaska
as the low center tracks near Nunivak Island through Thursday
night. Winds will ramp up through typical gaps and passes, with
the highest winds expected near Cold Bay this afternoon through
tonight, and out of Kamishak Gap into interior Bristol Bay late
tonight into Thursday morning.

A steady flow of southerly gale force winds bump up against the
northern edge of the Kuskokwim Bay. The resulting higher seas and
surf coinciding with high tide could raise water levels a few
feet above normal highest tide to the area, primarily from Kipnuk
to the mouth of the Kuskokwim River Thursday night into Friday.
A Special Weather Statement has been issued addressing this
concern; stay tuned to the forecast for any updates and/or
additional information.

The low is expected to exit to the north by the end of the week,
bringing a gradual end to winds and rain across Southwest.

&&

.LONG TERM FORECAST (Days 4 through 7: Saturday through Tuesday)...

A fall-like weather pattern continues to dominate the extended
period as a series of lows moves into Southern Alaska and the
Bering Sea through early next week. Confidence remains on the
higher end for stormy weather and for the general large-scale
flow, but confidence is lower for individual storm systems.

The extended period begins on Saturday with a ridge over Eastern
Alaska/the Gulf and broad troughing over the Bering Sea/Western
Alaska. A vigorous shortwave and its associated North Pacific
surface low will ride up along the ridge and move into Southern
Alaska, quick on the heels of another low that had moved through
on Wednesday and Thursday. This will be the moister of the two
storms as it directs an atmospheric river into Southcentral and
Southwest Alaska. We are confident that this atmospheric river
will bring periods of moderate to heavy rain, with less confidence
in overall rain intensity and amounts as the path of the
atmospheric river could shift. We'll be keeping a close eye on its
track, but current thinking is that the highest rain amounts will
occur in Prince William Sound.

Model agreement with individual systems declines by Sunday.
Broadly speaking, a low exits off Kamchatka Peninsula on Saturday
and moves into the Bering Sea by Sunday. There is some
uncertainty with how quick and how far southeastward it tracks.
Meanwhile, farther east, the question is whether a ridge in the
Eastern Gulf will subside and allow the jet stream/storm track to
move eastward. If not, Southcentral could face more rain as lows
continue to run up against the ridge and become forced northwards
into Mainland Alaska.

&&

SYNOPSIS AND MODEL DISCUSSION...BL
SOUTHCENTRAL ALASKA...SB
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...KM
LONG TERM...KC